U.S. patent application number 14/355268 was filed with the patent office on 2015-08-27 for scanning method and apparatus.
The applicant listed for this patent is JOHNSON MATTHEY PUBLIC LIMITED COMPANY. Invention is credited to Christopher Bowdon, Paul David Featonby, James Stephen Howstan, Peter Jackson, Kenneth James, Emanuele Ronchi.
Application Number | 20150241572 14/355268 |
Document ID | / |
Family ID | 47429104 |
Filed Date | 2015-08-27 |
United States Patent
Application |
20150241572 |
Kind Code |
A1 |
Bowdon; Christopher ; et
al. |
August 27, 2015 |
SCANNING METHOD AND APPARATUS
Abstract
The invention discloses a scanning method and apparatus suitable
for scanning a pipeline or process vessel in which a beam of gamma
radiation from a source is emitted through the vessel to be
detected by an array of detectors which are each collimated to
detect radiation over a narrow angle relative to the width of the
emitted radiation beam.
Inventors: |
Bowdon; Christopher;
(Kempston Bedfordshire, GB) ; Featonby; Paul David;
(Wylam Northumberland, GB) ; Howstan; James Stephen;
(Bishop Auckland County Durham, GB) ; Jackson; Peter;
(London, GB) ; James; Kenneth; (Cleveland, GB)
; Ronchi; Emanuele; (Cleveland, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
JOHNSON MATTHEY PUBLIC LIMITED COMPANY |
London |
|
GB |
|
|
Family ID: |
47429104 |
Appl. No.: |
14/355268 |
Filed: |
November 2, 2012 |
PCT Filed: |
November 2, 2012 |
PCT NO: |
PCT/GB2012/052737 |
371 Date: |
April 30, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61597354 |
Feb 10, 2012 |
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61597237 |
Feb 10, 2012 |
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61597272 |
Feb 10, 2012 |
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Current U.S.
Class: |
378/64 |
Current CPC
Class: |
G01T 1/2018 20130101;
G01N 2223/419 20130101; G01N 9/24 20130101; G01N 23/046 20130101;
G21K 1/02 20130101; G01N 23/06 20130101; G01T 1/164 20130101; G01N
23/083 20130101; G01N 23/087 20130101; G01V 5/005 20130101; G01N
23/18 20130101; G01V 5/0016 20130101; G01N 2223/5055 20130101; G01N
23/04 20130101 |
International
Class: |
G01T 1/20 20060101
G01T001/20; G21K 1/02 20060101 G21K001/02; G01N 9/24 20060101
G01N009/24 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 2, 2011 |
GB |
1118943.8 |
Nov 2, 2011 |
GB |
1118944.6 |
Jan 17, 2012 |
GB |
1200744.9 |
Claims
1. A method of scanning a structure to detect changes in density
between different parts of the structure comprising the steps of:
a) providing at least one source of gamma radiation, and a
plurality of detector units capable of detecting said gamma
radiation, each said detector unit comprising: i.a radiation
detector comprising a scintillator comprising a scintillating
material and having a detecting surface defined by its thickness t
and height h, wherein t.ltoreq.h at the detecting surface and
having a depth d perpendicular to the detecting surface at least
2t, and ii.a photodetector for detecting light emitted by the
scintillator in response to gamma radiation, and iii.a collimator
placed between the scintillator and the source of radiation; b)
causing said source unit to emit gamma radiation along a
predetermined radiation path towards said detector, wherein said
path passes through at least a portion of said structure; c)
measuring the number of photons of gamma radiation detected by each
one of said detectors; d) calculating a density value for each path
from the measurement of photons detected by the detector associated
with the respective path.
2. A method according to claim 1, wherein the plurality of detector
units are arranged in close proximity to each other in the form of
an arc having a radius centred on the centre of the structure to be
scanned.
3. A method according to claim 1, wherein the source unit and
detector units are mounted spaced apart on a support which provides
means for at least a portion of the structure to be scanned to be
located between the source unit and the detector units.
4. A method according to claim 1, wherein the source unit and
detector units are rotated around the structure the radius of
rotation having an origin within the structure.
5. A method according to claim 4, wherein guiding means are
provided to guide said rotation in a predetermined path.
6. A method according to claim 5, wherein said guiding means is
indexed.
7. A method according to claim 1 wherein 1 mm.ltoreq.t.ltoreq.5 mm,
10 mm.ltoreq.h.ltoreq.50 mm and 25 mm.ltoreq.d.ltoreq.75 mm.
8. A method according to claim 1 wherein the depth of the
collimator of each detector unit is in the range from 50-150
mm.
9. A method according to claim 1, wherein the end of the collimator
distal to the scintillator has an opening lying on the plane of a
tangent to a circle having the source as its origin.
10. A method according to claim 1, wherein at least one of the
collimators has at least one wall defining a channel which is
aligned with a radius of a circle having the source as its
origin.
11. A method according to claim 10, wherein each of the walls of
the channel is aligned with a different radius of said circle and
the opening of the channel is aligned to face directly towards the
source.
12. A method according to claim 1, wherein said photodetector is
mounted at an angle to the axis of the collimator and
scintillator.
13. An apparatus for scanning a structure to detect changes in
density between different parts of the structure comprising at
least one source unit comprising a source of gamma radiation and
shielding material arranged to restrict the emission of gamma
radiation from the source unit: a plurality of detector units
capable of detecting said gamma radiation, each said detector unit
comprising: i. a radiation detector comprising a scintillator
comprising a scintillating material and having a detecting surface
defined by its thickness t and height h, wherein t.ltoreq.h at the
detecting surface and having a depth d perpendicular to the
detecting surface at least 2t, and ii. a photodetector for
detecting light emitted by the scintillator in response to gamma
radiation, and iii. a collimator placed between the scintillator
and the source of radiation; and data processing means for
calculating a density value for each path from the measurement of
photons detected by the detector associated with the respective
path.
14. An apparatus according to claim 13, wherein said source unit
and said detector units are mounted on a support in a fixed spaced
apart relationship having means for rotating the source unit and
detector units around an arc having an origin located between the
source unit and detector units.
15. An apparatus according to claim 14, wherein said means
comprises guiding means for guiding the rotation in a predetermined
path.
16. An apparatus according to claim 15, wherein said guiding means
comprise more than one portion which are movable to form a
continuous guiding means.
17. An apparatus according to claim 13, wherein said collimator is
formed, at least in part, from a first material having a first
shielding capacity for gamma radiation, and a layer of a second
material, having a greater shielding capacity than the first
material, is positioned over at least one external surface of the
detector unit.
18. An apparatus according to claim 13, wherein the plurality of
detector units are arranged in close proximity to each other in the
form of an arc having an origin located between the source unit and
detector units.
19. An apparatus according to claim 13, wherein 1
mm.ltoreq.t.ltoreq.5 mm, 10 mm.ltoreq.h.ltoreq.50 mm and 25
mm.ltoreq.d.ltoreq.75 mm and the depth of the collimator of each
detector unit is in the range from 50-150 mm.
20. An apparatus according to claim 13, wherein the end of the
collimator distal to the scintillator has an opening lying on the
plane of a tangent to a circle having the source as its origin.
21. An apparatus according to claim 13, wherein at least one of the
collimators has at least one wall defining a channel which is
aligned with a radius of a circle having the source as its
origin.
22. An apparatus according to claim 21, wherein each of the walls
of the channel is aligned with a different radius of said circle
and the opening of the channel is aligned to face directly towards
the source.
23. An apparatus according to claim 13, wherein said photodetector
is mounted at an angle to the axis of the collimator and
scintillator.
Description
[0001] The present invention relates to a method of scanning a
structure to detect changes in density by means of detecting
radiation emitted by a radiation source by a radiation
detector.
[0002] Methods of imaging objects and animals by X-ray tomography
and positron-emission tomography are well-known, particularly in
the field of medical imaging for diagnostic purposes. U.S. Pat. No.
4,338,521 describes an X-ray scanner for use in computerised
tomography which has a detector comprising an array of detector
modules, comprising a plurality of photodiodes and a plurality of
scintillator crystals and a radiation beam collimator to direct
collimated radiation to the scintillator crystals. A fan-shaped
beam of x-rays from an x-ray source is directed through a patient
to be detected by the detector. The source and detector are rotated
around the patient to provide the data from which a tomographic
image may be constructed. In positron-emission tomography (PET) a
positron emitted by the decay of a radionuclide annihilates on
contact with a suitable electron, causing the emission of two gamma
photons of 511 keV in opposite directions. The detection of the
direction of the gamma photons enables the estimation of the
location of the annihilation event and thus the radionuclide within
the patient. The PET scanner therefore incorporates an array of
detectors which can detect gamma photons placed around the body of
a patient. An image of the relative concentration of the
radionuclide in the body may be constructed from the number of
photons detected at each detector.
[0003] Whilst these scanning methods are well-developed and have
become common for medical scanning, scanning a dense structure such
as a pipeline presents difficulties because the density of the
pipeline material is such that radiographic scanning must be done
using gamma radiation which is of sufficient energy to penetrate
and pass through the structure so that at least some radiation can
be detected after the beam has passed through the structure. Gamma
scanning of structures such as distillation columns is a standard
industrial diagnostic method for measuring changes in density at
different parts of the structure, for example to determine the
location and integrity of column trays or other internal structures
within the column. Normally this type of scanning is carried out
with a single gamma source placed adjacent the column to emit a
beam of radiation through the column and a radiation detector
placed on an opposed part of the column to intersect and measure
the radiation that has traversed the column between the source and
detector. The source and detector are normally moved so that
different parts of the column can be scanned. The use of many
different positions and more than one source or detector can
provide sufficient data for generating density maps, or tomographs,
of the structure which is scanned, although the resolution is
generally quite coarse. In order to generate higher resolution
density maps, information from a much larger number of radiation
paths through the structure must be used than is currently achieved
with conventional column scanning methods.
[0004] The inspection of pipelines to find flaws such as wall loss,
cracks or corrosion pitting is an application in which it would be
desirable to use radiation scanning. A known problem for the oil
and gas production industry is the inspection of pipelines located
underwater, in particular on the sea-bed. Inspection of the
interior of the pipeline by the use of pigs is not always possible,
for example when the pipeline has varying diameter. Inspection from
outside the pipe may be carried out by ultrasonic methods, although
this is not suitable for pipelines having an insulation or coating.
Gamma scanning can produce useful information about the density
through a cross-section of the pipe. In order to produce
information about the thickness of the walls of the pipe at
sufficiently high resolution to identify small flaws that may be
present in the walls of the pipe a large number of radiation paths
through the pipe need to be scanned. Furthermore, if a fan-shaped
radiation beam is to be used to scan the pipe, many of the
radiation paths pass through a chord of the pipe cross-section and
therefore through a relatively large amount of the pipeline wall
material, requiring a relatively high energy gamma source. In order
to detect the gamma radiation that has passed through the structure
it is necessary to use detectors of sufficient size and density to
stop the gamma photons so they do not pass through the detector
undetected. In order to maintain a high resolution the collimation
of the detectors has to be sufficient to significantly reduce
detection of gamma photons which have been scattered from a path
other than the direct path to a particular detector. The detectors
need to be small enough to provide good spatial resolution. A large
number of detectors is needed to achieve a reasonable measurement
time. The use of heavy collimation on a large number of detectors
necessitates a scanning apparatus which is very heavy and so
rotation of the apparatus in a controlled and precise manner around
a large pipeline becomes very difficult. When the pipeline is
horizontal, it becomes necessary to trench the pipe in order to
allow sufficient space in which to move a scanning apparatus and so
the use of large apparatus becomes expensive, particularly when the
pipeline is located sub-sea. All of these considerations bring
particular problems to the application of high resolution x-ray
tomography methods to the scanning of pipelines or other large
structures with high energy gamma radiation.
[0005] It is an object of the present invention to provide such a
method, although the method of the invention may be useful for
scanning structures other than pipelines and for use in locations
including both dry and sub-sea locations.
[0006] The present invention relates to a method of scanning a
structure to detect its physical properties. In particular the
invention relates to a method of scanning an elongate structure,
such as a pipeline, to detect changes in its material density which
may indicate variations in wall thickness caused by corrosion or
erosion or to deduce information about the contents of the pipeline
such as the build-up of deposits or the nature of fluid flowing
within the pipeline. Typically, the method and apparatus concerns
the measurement of density of a structure by means of detecting
radiation emitted by a radiation source by a radiation
detector.
[0007] According to the invention, we provide a method of scanning
a structure to detect changes in density between different parts of
the structure comprising the steps of: [0008] a) providing at least
one source of gamma radiation, and a plurality of detector units
capable of detecting said gamma radiation, each said detector unit
comprising: [0009] i. a radiation detector comprising a
scintillator comprising a scintillating material and having a
detecting surface defined by its thickness t and height h, wherein
t.ltoreq.h at the detecting surface and having a depth d
perpendicular to the detecting surface at least 2t, and [0010] ii.
a photodetector for detecting light emitted by the scintillator in
response to gamma radiation, and [0011] iii. a collimator placed
between the scintillator and the source of radiation; [0012] b)
causing said source to emit gamma radiation along a predetermined
radiation path towards said detector, wherein said path passes
through at least a portion of said structure; [0013] c) measuring
the number of photons of gamma radiation detected by each one of
said detectors; [0014] d) calculating a density value for each path
from the measurement of photons detected by the detector associated
with the respective path.
[0015] According to the invention, we provide an apparatus for
scanning a structure to detect changes in density between different
parts of the structure comprising: [0016] at least one source unit
comprising a source of gamma radiation and shielding material
arranged to restrict the emission of gamma radiation from the
source unit: a plurality of detector units capable of detecting
said gamma radiation, each said detector unit comprising: [0017] i.
a radiation detector comprising a scintillator comprising a
scintillating material and having a detecting surface defined by
its thickness t and height h, wherein t.ltoreq.h at the detecting
surface and having a depth d perpendicular to the detecting surface
at least 2t, and [0018] ii. a photodetector for detecting light
emitted by the scintillator in response to gamma radiation, and
[0019] iii. a collimator placed between the scintillator and the
source of radiation; [0020] and data processing means for
calculating a density value for each path from the measurement of
photons detected by the detector associated with the respective
path.
[0021] The apparatus of the invention is suitable for use in the
scanning method of the invention, in which a target structure is
scanned to detect changes in its shape or composition by means of
passing radiation emitted by a radiation source through the
structure and detecting radiation after it has passed through the
structure. The method works on the well-known principle that the
amount of radiation attenuated or scattered by an object is related
to the mass of material the radiation has passed through. By
measuring the amount of radiation detected through each selected
path through the target structure it is possible to calculate
and/or compare the density of the structure along one radiation
path with the density of the structure along a different radiation
path. By "density value" we mean a value which represents the
actual or relative density of the structure which lies on a
particular path from the source to a particular detector. The
density value may take the form of a number of counts of gamma
photons or a normalised, smoothed or comparative number of counts
of gamma photons. Alternatively the density value may be a value
calculated from the number of counts of gamma photons. The density
value may be expressed graphically, including as an image or part
thereof. The relative dimensions of the scintillator(s) of the
radiation detector of the invention allow a plurality of
scintillators to be placed in close proximity in order to achieve a
high degree of spatial resolution of detected radiation so that
characteristics of small portions of the structure may be detected
with high precision. The method is particularly useful for scanning
a regular structure such as a pipe, although the method and
apparatus may be used for scanning other types of structure. In a
particular embodiment of the invention, the scanning method is a
method of detecting changes in the density of the wall of a
pipeline. Use of this method enables flaws such as voids, cracks,
scale, gas hydrates or thinning to be detected. The change in
density may be detected relative to adjacent portions of the pipe
wall or relative to a reference value generated from a model
pipeline or a calculated value.
[0022] In the method of the invention, an array of detector units
is mounted opposite at least one source of gamma radiation such
that the radiation is emitted in the direction of the detecting
surfaces. The target structure to be scanned is capable of being
interposed between the source and detector unit so that the
radiation emitted by the source can pass along a plurality of paths
through a portion of the structure and impinge upon the detecting
surfaces. The source and detector unit may be moved relative to the
target structure (or vice versa) in order to scan different
portions of the structure. The principal benefit of using an array
of detectors is that different paths through the structure may be
scanned simultaneously. Each of the paths has the shape of a
frustum, having the source at the apex and the detecting surface of
a detector at the base. Each detector in the array defines a
different path through the structure so that the number of paths
which may be scanned simultaneously is equal to the number of
detectors in the array. The number of detectors in a detector array
may vary from fewer than 10 to more than 100, e.g. up to 150,
depending on the application for which the scanning method is to be
used. In practice, the mass of shielding material required to
shield and collimate a large number of detectors may provide a
practical upper limit to the number that can be used.
[0023] The source unit and detector unit may be mounted on a
support in fixed relationship to one another or the detector unit
may be movable relative to the source unit. It is greatly preferred
that the source unit and detector unit are mounted in a fixed
relationship when the apparatus is in operation. This enables the
apparatus of the invention to provide a precise and fixed alignment
of source and detector units so that modulation of the counts
measured by the detectors can be attributed solely to the materials
between the source and detector through which the radiation path
passes. In this way, very small differences in the density of such
materials can be detected, allowing the detection of small flaws or
changes in the thickness of a pipeline wall. The source and
detector unit are preferably mounted so that the detecting
surface(s) of each of the detectors form a tangent to an arc having
the source at its origin. The plurality of detector units are
arranged in close proximity to each other. It is preferred that the
array of detector units is arranged in the form of an arc having a
radius centred on the centre of the object or structure to be
scanned. The design of the detector unit preferably minimises the
total distance on each detection path through each detector unit in
order to make the array of detector units as compact as possible,
whilst maintaining sufficient depth of the collimation and a
detector for efficient detection of gamma photons on each path.
[0024] In a preferred form of the invention, the source unit and
detector units are mounted on a support which provides means for
the structure to be scanned, or a portion thereof, to be located
between the source unit and the detector unit. The support
maintains the source unit and detector unit in a spaced apart fixed
relationship. The support therefore includes means for mounting at
least one source unit and means for mounting a plurality of
detector units on the support. The support may comprise an elongate
portion or "arm" having first and second opposed ends to which said
detector unit and support unit may be mounted or joined. The means
for mounting a detector unit comprises a detector housing joined to
the support. The support, source unit, and/or detector housing may
be formed as a unitary component or from separate components which
are joined together. The support must be sufficiently strong to
withstand supporting and moving the detector and source units
without deformation and sufficiently rigid to maintain a precisely
fixed relationship between the source unit and detector housing,
including any detector units housed therein. One suitable material
for the support comprises an aluminium alloy, which may be machined
by known methods to form the required shape for the support.
[0025] The detector housing is shaped to house one or more detector
units and to secure such units so that they do not move
unintentionally, during operation of the apparatus. It is an
important feature of the preferred apparatus that the detector
units can be maintained in a fixed relationship to the source
during use in a scanning method. The detector housing may be of
such size and shape to house several detector units at the same
time, for example from 2-100 units. The detector housing may
include means to house a detector unit in more than one position
within the housing. The means may simply comprise a detector
housing having sufficient space to house a detector unit in more
than one location within the housing. Means, such as guide rails or
a motor may also be included to move one or more detector units
from a first location within the housing to a second location
within the housing. A practical limit on the resolution of a
scanning method using an array of detectors is that the spacing
between each one must be sufficient to allow a minimum required
amount of shielding to ensure that each detector is adequately
shielded from photons impinging on a neighbouring detector. Even
when highly dense alloys are used for detector shielding, we have
found that a practical limitation on detector spacing is
approximately 1.degree. of arc. In one embodiment of the apparatus,
the detector housing is of such a size as to allow a detector unit
to be housed in at least two positions, offset from each other by a
distance which is a fraction of the distance between the detectors.
When the fraction is 0.5 of the detector spacing distance (0.5 s),
the resolution of the apparatus may be doubled by carrying out a
first scan when the detector array is in a first position in the
housing and then repeating the scan when the detector array is in a
second position in the housing which is offset from the first
position by 0.5 s. If additional positions are provided, and or,
the angular distance between them is reduced, additional scans can
provide additional data to enhance the resolution of the scan. The
detector may be moved between any of the at least two positions,
for example by operation of a powered switch operated by a
solenoid. The provision of means to lock the detector array
securely in a single position whist scanning is greatly preferred.
Such means may comprise a sprung pin or boss engaging with an
indexing hole in each of the desired positions.
[0026] In a particularly preferred form, the apparatus comprising
support, detector housing (including any detector unit therein) and
source unit may be moved laterally and/or rotationally, relative to
the structure and means are provided to effect such movement.
Preferably the source unit and detector units are rotated around
the structure such that the radius of rotation has an origin within
the structure, for example the origin may be approximately at the
geometric centre of the structure in the plane of rotation. The
means for said movement may include motorised or manual impulsion
and guiding means such as rails, tracks, guide channels or locating
indicators, to guide the path of rotation. Preferably the apparatus
is provided with at least one track or rail, shaped to conform to
at least a part of the structure to be scanned. For pipeline
scanning, for example, one or more arcuate tracks may be provided
so that the apparatus may be moved along the tracks, for example by
means of a worm drive or a stepper motor turning a splined
drive-wheel, gear or cog to rotate the detector housing and source
around the circumference of the structure. In a preferred form, the
guiding means is indexed, for example by providing indentations
into which teeth of a drive cog may engage to effect movement of
the scanning apparatus. The provision of indexed movement may
provide a predetermined number of scanning locations at known
angular positions around the structure. Preferably a means is
provided to rotate the detector units and source around a
circumference of the structure to be scanned. In the case of a
cylindrical object, such as a pipeline, the detector units and
source are rotated around the circumference of the pipeline. The
scanning method is carried out at a plurality of radially offset
positions around the structure so that density data may be acquired
at a variety of angles through the structure.
[0027] The guiding means, e.g. tracks, may extend partially or
entirely around the pipeline. It is preferred to move the source
and detector unit continuously around the structure in order to
avoid the problems, such as damage to the apparatus or slippage of
the scanning system, associated with successively accelerating and
braking the apparatus. More than one scan may be required to gather
sufficient data to determine the structure properties, although the
number of scans and the scanning time is dependent upon the density
and mass of material through which the radiation must travel from
the source to the detector units. Preferably a continuous
rotational movement around the structure at a relatively low rpm,
for example at from about 1 to about 20 rpm, especially from 1-10
rpm is maintained during the scanning operation. Therefore in a
preferred apparatus means such as a continuous track are provided
to enable such movement. The guiding means may be provide in more
than one part, which, following deployment of the apparatus, are
brought together and optionally joined, to form the desired length
of track for scanning. The source unit, detector unit, support and
guiding means may all be housed within an enclosure which is
capable of surrounding at least a part of the structure. The
enclosure may have an open position in which it may be positioned
around the structure and a closed position in which it is capable
of scanning the structure. The enclosure may take the form of a
hinged pair or set of jaws which may be clamped to the structure to
be scanned.
[0028] Power may be recovered from the movement of the apparatus by
means of a dynamo, or similar, which may be then used to help power
the detectors or other operating systems of the apparatus.
[0029] Movement of the apparatus may also involve lifting and/or
sliding the apparatus manually or by mechanical means, for example
by means of a remotely operated vehicle (ROV). An ROV may be
preferred to deploy and move the apparatus when deployed in remote
or underwater locations. Linear movement, for example parallel to
the axis of a pipeline or a vessel, may be achieved by means of a
crawler mechanism or using a track or rails, or alternatively by
external means such as a lifting apparatus or ROV. The movement
means may include indexing, for example at a particular angular
separation in order to provide a predetermined number of scanning
locations at known positions around the structure. For an
application such as scanning a pipeline, the movement may be
controlled by means of a programmed electronic control unit, for
example to execute a predetermined timed movement or set of
movements of the source and detector units relative to the
pipeline. The movement may be rotational to scan around the
circumference of the pipeline and/or lateral to move axially along
the pipeline.
[0030] The apparatus may comprise means to support the apparatus in
proximity to the structure to be scanned. Such means may comprise
clamps, which are capable of engaging the structure and supporting
the scanning apparatus in one or more positions on the structure.
The clamps may be manually operated but mechanically operated
clamps are preferred.
[0031] The scintillating material is selected according to the
properties of the radiation which is to be detected and the
conditions in which the detector is deployed. In principle, any
suitable scintillating material may be selected and many materials
are known and marketed for the detection of radiation. A high
density material provides a greater capacity to stop radiation in a
given volume and consequently the scintillator can be made smaller
than would be possible for a lower density material. A small
scintillator is more stable, for example it is less likely to
exhibit a temperature differential between different parts of the
crystal. Smaller crystals transmit light more effectively requiring
lower-powered photo-detectors to be used. Of importance for the
present application a small scintillator can have a small detecting
surface and so radiation travelling along a narrow path can be
detected without a significant amount of incident radiation from
background or scattered radiation from the same or different paths.
For the detection of gamma radiation, it is preferred to use a
dense inorganic material so that the incident photons may be
stopped using as small a detector as possible. Scintillating
materials having a density>5 and a high Z-number (atomic number)
are preferred. It is preferred that the scintillator(s) have a
depth and density that enable them to stop >80% of gamma photons
of energy of 662 keV. For use in applications requiring resistance
to environmental conditions, especially moisture, a non-hygroscopic
crystal scintillator should be selected. Especially preferred
detectors for use with gamma radiation include BGO (bismuth
germanate), CdWO.sub.4, LaBr.sub.3(Ce), LYSO (lutetium yttrium
oxyorthosilicate--cerium doped), LSO (lutetium
oxyorthosilicate--cerium doped) and CeF.sub.3 (cerium fluoride).
When a mechanically rugged detector is required, a crystal having
no cleavage planes may be preferred in order to increase its
resistance to shattering following a thermal or mechanical
shock.
[0032] Each scintillator has a detecting surface, which, in use is
arranged to intersect the radiation path so that the radiation
impinges upon the detecting surface. Other surfaces of the detector
which are not arranged to be detecting surfaces will be referred to
as non-detecting surfaces. Although any part of a scintillator is
normally capable of detecting photons, the designation in this
specification of detecting surfaces and non-detecting surfaces
refers to the arrangement of the scintillator in the detector unit
for detecting radiation from a source. The scintillator also has a
surface through which light generated by the scintillator in
response to photons impinging upon the detecting surface leaves the
scintillator. This surface is referred to herein as the collecting
surface. The collecting surface is arranged in optical
communication with the photodetector. The collecting surface can be
in contact with the photodetector or it may be separated therefrom
by one or more light transmitters, in the form of a window, lens,
optical fibre, light pipe or optically coupling material etc. made
from a material which transmits the light generated by the
scintillator to the photodetector. The collecting surface of the
detector may have a similar cross-sectional area and shape to that
of the photodetector window or it may be different. The detector
itself may act as a light guide to pass a substantial proportion of
the light generated in the scintillator to the photodetector. In
this context, the use of the phrase "substantial proportion" means
all of the light generated in the scintillator is passed to the
photodetector, save for a proportion of light that is
unintentionally lost due to the efficiency of the light
transmission being less than 100%.
[0033] Each of the detectors comprises a scintillator, normally
supported in a suitable position so that the detecting surface
intersects a path of radiation emitted by the source at a
particular distance from and a particular angle to the radiation
source. It is a particular feature of the invention that the
detector can substantially reduce the detection of scattered
radiation and increase the precision with which radiation emitted
by a source along a particular linear path is detected. The
provision of a detector having an elongate shape in which
t.ltoreq.h, more preferably <0.5 h, at the detecting surface
enables the detectors to be located in close proximity so that
spatial resolution of each detector is high. The depth of the
detector contributes to the stopping efficiency of the detector so
that a detector having a depth d perpendicular to the detecting
surface at least 2t, more preferably at least 5t, especially
>10t is preferred in order to stop and measure energetic
photons.
[0034] The smallest dimension of the detecting surface of the
scintillator is preferably between about 1 mm and about 10 mm. The
smallest dimension is defined to be the thickness t of the
material. More preferably, 1 mm.ltoreq.t.ltoreq.5 mm and in a
preferred embodiment t is about 5 mm. Preferably the detecting
surface is generally rectangular so that the area of the surface is
defined as t.times.h, where h is in the range 5-100 mm. More
preferably, 10 mm.ltoreq.h.ltoreq.50 mm and in a preferred
embodiment h is about 25-40 mm. The depth, d of the scintillator is
in the range 10-100 mm. More preferably, 25 mm.ltoreq.d.ltoreq.75
mm and in a preferred embodiment d is about 40-60 mm.
[0035] A material which is impermeable to the radiation may cover a
part of the detecting surface of the scintillator to delimit the
portion of the detecting surface on which radiation may impinge.
The collimator may overlap and cover one or more edges of crystal
by up to about 5 mm.
[0036] The detection of scattered photons is preferably further
reduced by preventing the detectable radiation from impinging upon
the surfaces of the detector which are not detecting surfaces. This
may typically be achieved by covering the non-detecting surfaces,
except for the portion of the collecting surface in optical
communication with the photodetector, with a material which
prevents transmission of the radiation to the non-detecting
surfaces. In a preferred embodiment the detectors are surrounded by
shielding material so that all of the non-detecting surfaces,
except for the portion of the collecting surface in optical
communication with the photodetector, are protected from radiation.
By shielding material we mean a material which is highly
attenuating to the radiation which is to be detected by the
detector. Typically, a shielding material for protection from
ionising radiation such as gamma radiation includes lead and heavy
metal alloys. Such materials are well known to persons skilled in
the art of designing radiation detectors and nucleonic
instruments.
[0037] When the scintillator is thin, scintillation light generated
as a result of the interaction of a gamma photon with the
scintillation material is likely to be internally reflected several
times before it enters the photodetector. Since each reflection may
be less than 100% efficient, the capacity for multiple reflections
provides multiple opportunities for loss of light and thus a
decrease in the detection efficiency of the detector. It is
therefore preferred to provide the non-detecting surfaces with
means to reflect light internally within the detector. Preferably
the non-detecting surfaces are coated with a super-reflective
coating, capable of reflecting at least 95% of the light within the
scintillator and more preferably at least 98% of that light.
[0038] When the detector unit comprises more than one detector,
deployed in the form of an array of detectors, a preferred
embodiment of the invention comprises a block of shielding material
(a "detector block") having openings extending inwardly from a
surface of the block, each opening containing a detector, the
detecting surface being accessible to radiation from outside the
block. A portion of the detecting surface may be covered by
shielding material for the purposes of delimiting the area of the
detecting surface or for mechanically retaining the detector within
the opening. The non-detecting surfaces of the detector may
optionally be enclosed partially or wholly within the opening and
covered by the shielding material. The detector block includes
means by which the collecting surface of the scintillator(s) may be
brought into contact with a photodetector or a light transmitter.
Such means may take the form of an open passage through which the
scintillator extends so that the collecting surface is accessible
to the photodetector or light transmitter.
[0039] The precision of the detector is increased by providing
collimation means for restricting the path along which radiation
may travel to the detecting surface. The collimation means
comprises a collimator formed from a shielding material and
arranged so that radiation travelling towards the detecting surface
from selected directions may contact the detecting surface whilst
radiation travelling from non-selected directions is excluded from
the detecting surface. In this way, only radiation travelling along
selected paths from a radiation source to the detector may be
detected. The collimation may be arranged so that radiation from
one or more selected radiation sources is detected. Suitable design
of the collimation can significantly reduce the detection of
scattered photons, which are usually deflected from the path along
which they were emitted by the source. Alternatively, the
collimation may be designed so that scattered photons and other
secondary radiation is detected preferentially. In a preferred
embodiment the collimation means comprises a block of shielding
material having a channel, or preferably a plurality of channels
extending through. The collimator block comprises a plurality of
channels, each channel being formed through the block and
corresponding in position to one of the detectors in said array.
Each channel is shaped to define the path of radiation which is to
be detected by each scintillator. Each channel has an opening at
the end proximal to the scintillator which is preferably mounted
over the detecting surface of the scintillator so that the
detecting surface, or a portion of it, is within the opening of the
channel. The end of a channel distal to the scintillator is open to
allow radiation to enter the channel and travel to the
scintillator. The opening preferably lies on the plane of a tangent
to a circle having the source as its origin. The area of the distal
opening defines the maximum useful area through which radiation can
pass to the detecting surface. The channel walls are normally
straight. The length of the channel(s) is determined according to
the requirements of the detector and the energy of radiation
emitted by the source. A longer channel reduces the detection of
scattered or reflected radiation more than a shorter channel and so
the resolution of detection of a particular path of radiation is
higher. The length of the collimation channels may be determined by
the skilled person according to the type of radiation which is to
be collimated, in accordance with known principles of physics.
Generally for collimating radiation from a caesium source (which is
a preferred source for use in the method and apparatus of the
invention), a collimation depth of at least 50 mm should be used. A
cobalt source requires more collimation and generally a depth of at
least 75-80 mm would be used. Americium emits less energetic gamma
radiation and requires only about 20 mm of collimation depth.
Americium may be used in some applications but would not be
suitable for use in scanning steel pipelines, which is a preferred
application. The depth, d of the collimator channels is preferably
in the range from 30-150 mm. More preferably 50 d 50 and, for use
with a caesium source suitable for scanning large pipelines d is
most preferably about 80-120 mm.
[0040] The cross-section of the channel may be any convenient
shape, although it is preferred that the channel has the same shape
and orientation as the detecting surface. Often, the channel has a
generally rectangular cross section. The shape and/or size of the
channel cross-section may change along the length of the channel,
or they may remain substantially constant. In a preferred
embodiment, at least one of the collimator channels has at least
one wall defining the channel which is aligned with a radius of a
circle having the source as its origin. Preferably each of the
walls of the channel is aligned with a different radius of said
circle so that the opening of the channel is aligned to face
directly towards the source. Preferably, in such an arrangement the
end of the collimator distal to the scintillator has an opening
lying on the plane of a tangent to a circle having the source as
its origin. In this way the detection of photons travelling in a
straight line from the source, through the target structure along
the collimator channel to the detector may be maximised for any
given area of detecting surface. More preferably all of the
collimator channels have at least one wall and preferably all of
their walls, aligned with the radius of a circle having the source
as its origin. In such an arrangement the walls of the collimator
channels are not parallel to each other and all of the channels
face the direction of the source. When this alignment of the
collimator channels is adopted, and the array of detector units is
arranged in an arc having an origin which is not the source, at
least some the collimator channels do not extend in a direction
which is perpendicular to a tangent to that arc. This is a
preferred arrangement for scanning a cylindrical structure such as
a pipeline. In order to produce the collimator channels having this
preferred alignment, it is preferred to form each channel in a
block of shielding material by means of a machining method. For
this reason the use of plates of shielding material, e.g. steel
plates, of the type found in the detector units of x-ray tomography
apparatus (for example as described in U.S. Pat. No. 4,338,521) is
not preferred.
[0041] In one embodiment of the apparatus of the invention, the
detector unit comprises a collimator block and a detector block,
joined together such that the proximal end of each channel is in
register with the detecting surface of a detector. The detector
block and collimator block are joined together so that the
connection between them does not allow radiation to impinge on the
detecting surface of a detector which has not traveled through a
channel in register with the detecting surface. It may be possible
to form the detector block and the collimator bloc from a single
piece of shielding material but it is normally easier to
manufacture them separately and then join them together.
[0042] The collimator block may be formed from a dense shielding
material such as lead or a heavy alloy which attenuates gamma
radiation. Alternatively, the collimator block may be formed, at
least in part, from a less dense material, such as steel for
example, which provides less shielding but which is not as heavy as
the more dense shielding materials such as lead or heavy alloy. In
one version of such a collimator, collimator channels are formed
from a first material, such as steel, and a layer of a second
material, such as a heavy alloy, having a greater shielding
capacity than the first material, is positioned over at least one
external surface of the detector unit. In this way the detector
unit may be better protected from the impact of scattered radiation
from selected directions than from other directions. In practice,
it is possible to determine, by calculation and/or modelling, the
probability at which gamma radiation scattered from particular
angles will impinge upon the detector unit. This information may
then be used to provide more shielding over those surfaces of the
detector unit at which scattered gamma photons are more likely to
contact the detector units. More shielding can be provided either
by using a more dense material or by increasing the thickness of
the shielding material. One advantage of providing different
shielding at different parts of the detector unit, or forming the
detector unit from different materials is that the weight of the
detector unit can be reduced whilst the shielding and collimation
of the detectors is substantially maintained. A further advantage
of using a material such as a steel to form at least a part of the
collimator is gained if the material has greater structural
strength than a traditional dense shielding material such as lead
or heavy alloy so that less structural support must be used to
support the collimator block.
[0043] The photodetector may be a photodiode, photomultiplier tube
(PMT) or other suitable light detecting device. Currently, PMTs are
preferred to photodiodes because they are more sensitive to very
low levels of light, although the use of other photodetectors, such
as silicon photomultipliers or avalanche photodiodes may become
preferable as technology develops. The photodetector generates an
electrical signal in response to light entering it through an
optical window. The wavelengths detected by the photodetector
should be matched as far as possible to the wavelengths generated
by the scintillator to maximise the detection efficiency. Normally
a photodetector is provided for each scintillator so that the
amount of radiation detected by each scintillator can be measured
independently of the other scintillators.
[0044] The photodetectors are held in position by attachment means
such as a clamp or mounting. When more than one photodetector is
present, they may be mounted in fixed positions within a mounting
block. The mounting block is formed from a material which is
impermeable to light and to any other radiation which is likely to
affect the signal produced by the photodetector. The photodetector
is mounted with its optical window optically coupled to a
collecting surface of the scintillator. The photodetector may be
coupled using an optically coupling adhesive. Selection of a
suitable optical coupling material such as an adhesive having some
resilient elastic properties can provide the detector unit with
some resistance to the effects of vibration or impact shock.
Normally the photodetector is adjacent the scintillator, but it may
be physically separated from the scintillator if light transmitting
means are provided to transmit light from the scintillator to the
photodetector. In that case it is important that the efficiency of
the light transmission is as high as possible.
[0045] The photodetector may be in a coaxial relationship with its
respective scintillator and collimator. Alternatively, the
photodetector may be mounted at an angle to the axis of the
collimator and scintillator, for example at an angle of between
about 45 and 100 degrees to that axis, especially about 90.degree..
One advantage of mounting the photodetector at an angle to the axis
of the scintillator and collimator is that the total depth of the
detector unit may be reduced compared with a detector unit in which
the photodetector is mounted coaxially. Reducing the depth of the
detector unit helps to minimise the space needed around the target
structure to carry out a scan and this can allow scanning in
restricted spaces and/or minimise the need for trenching a pipeline
prior to scanning.
[0046] In a preferred embodiment of the invention an array of n
detector units is provided, comprising an array of n radiation
detector comprising:
n scintillators, n photodetectors, each photodetector being
optically coupled with a respective scintillator, a detector block
made of highly attenuating material incorporating a plurality of n
channels extending through the detector block from a first surface
to a second surface, each channel being sized to accommodate a
single scintillator, and a collimator block comprising a block of
shielding material having n channels extending therethrough, and
wherein the collimator block is joined to the detector block so
that each channel is in register with a scintillator. wherein each
scintillator is located within a channel in the detector block,
where n=an integer in the range from 2-150.
[0047] Each detecting surface preferably forms a tangent to an arc
of a circle having a radiation source as its origin. In one
embodiment, each detector surface forms a tangent to the surface of
a part of a sphere having the radiation source as its origin.
[0048] The source unit comprises a source of penetrating radiation,
a source-holder and a collimator. The collimator and source-holder
may be combined. The collimator is formed of a material which is
highly attenuating to the radiation emitted by the source and is
normally formed of a heavy alloy material of the type known and
commonly used for shielding radiation of the appropriate energy and
type. The collimator is located and adapted to limit the radiation
emitted by the source unit to a predetermined beam shape and
direction. Preferably the radiation beam is shaped by the
collimator to form a fan, cone or frusto-cone, or sector in each
case having the source as origin. A preferred beam shape is a
cylindrical sector, i.e. a sector having a thickness rather than
being planar. Preferably the beam is collimated to provide a beam
area at the location of the detector(s) which has the same general
shape and area as the combined detecting surface(s) of the array of
detectors. In the preferred form of the apparatus, the source unit
is mounted on a support, preferably in the region of an end of an
elongate support.
[0049] The radiation source is selected by the transparency to the
radiation of the material(s) to be measured, e.g. a vessel and/or
its contents (i.e. the attenuation coefficient of the medium) and
the availability of suitable sources and detectors. For scanning
large solid structures such as process vessels and pipelines,
suitable sources of gamma include .sup.80Co and .sup.137Cs,
.sup.133Ba, .sup.241Am, .sup.24Na and .sup.182Ta, however any
gamma-emitting isotope of sufficient penetrating power could be
used, and many such are already routinely used in density gauges,
such as those used as level measurement devices. Usually, the
half-life of the radioisotope used will be at least 2, and
desirably at least 10, years. The half-lives of the radioisotopes
mentioned above are: .sup.137Cs gamma about 30 years, .sup.133Ba
about 10 years and .sup.241Am about 430 years. Suitable sources
generally emit radiation at energies between about 40 and 1500
keV.
[0050] The source unit may include one or more than one source. The
scanning method may utilise more than one source unit if
required.
[0051] The apparatus further comprises a signal/data processor for
operating on the electrical signal from the detectors in the
detector unit(s) and a controller to control the operation of the
apparatus. Signals representative of the counts of photons detected
by the scintillators are processed by the data processor. The
signal may be subjected to smoothing or stabilisation algorithms,
averaged or otherwise operated on according to standard practices.
A data processor may perform calculations based on the signal from
the radiation detector or from a signal processor if present. The
data processor may output information concerning the amount of
radiation measured over a time interval, or it may further
calculate derived properties of the scanned structure, usually in
the form of a bulk density or a change in bulk density between
radiation paths through the structure. The scanning method is
carried out at a plurality of radially offset positions around the
structure so that density data may be acquired at a variety of
angles through the structure and a tomography algorithm may be used
to provide information about the changes in density at different
paths through the structure. In a preferred form the data from the
detectors is operated on by the data processing unit using
tomography algorithms in order to produce a graphical
representation of the density or composition of the structure along
different paths. The data processor may contain a calibration or
information concerning the radiation source. The data processor
output is may be connected a display or a (optionally wireless)
transmission means so that a signal can be sent from the apparatus
to a remote location. Alternatively a signal comprising data from
the radiation detector itself may be sent, for processing at a
remote location. A power supply is provided to power the
photodetectors, data processor and control electronics and also to
power motors for moving the apparatus.
[0052] In use in the scanning method of the invention, the
apparatus is deployed so that the source unit and detector units
are positioned in relation to the structure to be scanned so that
one or more radiation paths from the source to detectors in the
detector unit pass through the desired portion of the structure.
The amount of radiation, in the form of counts, is measured by the
detector in each detector unit deployed in the apparatus. The
scanning method is carried out at a plurality of radially offset
positions around the structure so that density data may be acquired
at a variety of angles through the structure. The apparatus may
then be moved to a different location or orientation with respect
to the structure and the measurement is repeated. In this way a
record of the attenuation to radiation through each radiation path
through the structure may be gathered and used to calculate the
location of changes or to build a representation of the structure
and its contents. Information such as changes in density which may
highlight flaws or other features within the structure can be
obtained from the data gathered from the detectors using data
analysis tools known for use in tomography methods.
[0053] For operation underwater, it is preferred to increase the
buoyancy of the apparatus by means of a buoyancy block. If used,
the buoyancy block may be attached to the apparatus by means of a
flexible attachment so that the flotation force produced can be
balanced during movement of the apparatus. Additionally or as an
alternative, spaces within the apparatus may contain a foam
material in order to provide positive buoyancy to the apparatus.
Parts of the apparatus may be coated in a resilient foam material,
again for the purpose of providing buoyancy and also to protect the
apparatus from physical damage, such as impact damage, and
environmentally induced damage such as corrosion.
[0054] The invention will be further described with reference to
the attached drawings, which are:
[0055] FIG. 1: a schematic view of a scintillator suitable for use
in the scanning method and apparatus of the invention
[0056] FIG. 1A: A view of the scintillator of FIG. 1 from direction
A.
[0057] FIG. 1B: A schematic view of an alternative scintillator
suitable for use in the scanning method and apparatus of the
invention.
[0058] FIG. 2: a schematic view through a section of a detector
unit.
[0059] FIG. 3: a schematic view through a longitudinal section of a
detector unit
[0060] FIG. 4: a schematic view of a detector block forming part of
a radiation detector according to the invention.
[0061] FIG. 5: a schematic view of a photomultiplier mounting block
forming part of a radiation detector according to the
invention.
[0062] FIG. 6: a schematic view of a collimator block forming part
of a radiation detector according to the invention.
[0063] FIG. 5: a schematic sectional view of a radiation detector
according to the invention.
[0064] FIG. 7: a schematic view through a section of an alternative
detector unit
[0065] FIG. 8: a view of a front elevation of the detector unit of
FIG. 7.
[0066] FIG. 9: a diagrammatic view of a preferred arrangement of
the apparatus.
[0067] FIG. 10: a schematic view of an array of detector units for
an apparatus according to the invention
[0068] FIG. 11: a schematic view of a part of an apparatus
according to the invention.
[0069] FIG. 12: a schematic elevation view of an apparatus
according to the invention
[0070] FIG. 13: a schematic perspective view of the apparatus shown
in FIG. 12;
[0071] FIG. 14: a schematic elevation view of an apparatus
according to the invention, and
[0072] FIG. 15: a schematic elevation view of an apparatus
according to the invention.
[0073] FIGS. 1 and 1A show a BGO scintillator crystal 10 having a
thickness t of 5 mm, a height h of 30 mm and a depth d of 75 mm.
The detecting surface 12 is opposite the collecting surface 14. All
surfaces of the crystal except the detecting and collecting
surfaces are coated in a highly reflective coating. FIG. 1B shows
an alternative scintillator crystal.
[0074] FIG. 2 shows, a transverse cross-section through a detector
unit 30, comprising a block of heavy alloy 16 which is highly
attenuating to radiation, of the type used as shielding material
for gamma radiation. The block has collimation channels 18
extending from the front face to the opposed rear face of the
block. In use, a scintillator crystal 10 is housed the block, with
the detecting surface of the crystal at the front face of the block
and the collecting surface 24 optically connected to PMT 20. PMT
20A is connected to an adjacent crystal 10 (not shown), and is
shown to demonstrate the packing of the PMTs within the detector
unit. The detector unit comprises nineteen detectors, each
comprising a crystal 10 and a PMT 20 and mounted in register with a
collimation channel 18 in the detector unit block.
[0075] FIG. 3 shows a longitudinal section through an assembled
radiation detector comprising a collimator block 40, a detector
block 20 and a PMT mounting block 30, each shown individually in
FIGS. 4-6. The blocks are mounted together so that channels 46, 36
and 26 are all in register, together forming channels extending
from the front face of the collimator block to the rear of the PMT
mounting block. A scintillator 10 is housed within channel 26 and a
PMT 50 is housed within channel 36. The PMT may be connected to
electronic data processing and control apparatus by means of
connectors accessible from the rear of the channel 36. Channels
46a, 36a and 26a and the scintillator 10a and PMT 50a are shown in
dashed outline because they are not in the same plane as the
respective channels and components in solid outline. In the
embodiment shown, the longitudinal axes of the channels form an
angle of between 1 and 2.degree. with the longitudinal axis 51 of
each block 20, 30 and 40.
[0076] FIG. 4 shows a detector block which comprises a rectangular
block of heavy alloy 21 which is highly attenuating to radiation,
of the type used to as shielding material for gamma radiation. The
block has channels 26 extending from the front face 22 of the block
to the opposed rear face 24. Blind sockets 28 are provided for
locating and mounting a collimator block. The channels 26 are sized
to house a scintillator crystal. In use, a scintillator crystal is
housed in each channel 26, with the detecting surface of the
crystal at the front face 26 of the block and the collecting
surface at the rear face 24. FIG. 5 shows a photomultiplier
mounting block 30 which comprises a rectangular block of white
plastic material such as polytetrafluoroethane. Channels 36 extend
from the front face 32 of the block to the opposed rear face 34.
The channels are each of an appropriate size to house a small
photomultiplier tube. The channels are positioned in the block in
such a way that each opening at the front face of the block 30
butts against the collecting surface 14 of a scintillator crystal
mounted in an adjacent detector block when the rear face 24 of the
detector block is placed against the front face 32 of the PMT
mounting block. Blind sockets 38 are provided for locating and
mounting to the detector block. FIG. 6 shows a collimator block
comprising a rectangular block of heavy alloy 40 which is highly
attenuating to radiation, of the type used to as shielding material
for gamma radiation. The block has channels 46 extending from the
front face 42 of the block to the opposed rear face 44. Blind
sockets 48 are provided for locating and mounting a detector block.
The channels 46 have a width and height which is slightly less than
the width and height of the scintillator housing channels in the
detector block. In an assembled radiation detector, rear face 44 of
collimator block 40 is mounted against the front face 22 of
detector block 20 such that channels 46 are in register with
channels 26. In FIG. 10, a detector unit consisting of 19
collimator channels, scintillators and PMTs is shown, in which the
collimator channels are formed into a single block being spaced
apart and angled from each other by an angle of about 1 degree of
arc.
[0077] FIGS. 7 and 8 show an alternative arrangement for a detector
unit. In FIG. 7, the block 60 forming the collimator 62 and holding
the scintillator 64 and PMT 66 is formed from a stainless steel.
The PMT is mounted out of alignment with the radiation direction in
order to reduce the total depth of the detector unit. The direction
of radiation is indicated by the arrow. FIG. 8 shows an elevation
from the direction of the arrow. Layers 68 and 69 of a dense heavy
alloy shielding material are positioned above and below the steel
block 60. This material provides additional shielding for the
detectors from scattered radiation impinging on the detector
unit.
[0078] FIG. 11 shows a support 70 joined rigidly at one end to a
generally arcuate shaped detector housing 72, all formed of an
aluminium alloy, and at the other end to a source unit 74. An
arrangement of the source and detector units is shown in FIG. 9.
The direction of three collimator channels is illustrated in order
to show that they align with the direction of the source and are
not aligned with the radius Rt of the structure. The source and
detector housing is arranged to rotate about a central point on the
structure on a path having a radius Rt. The source unit comprises a
caesium source of gamma radiation 78 surrounded by heavy alloy
shielding material 76 including a slot for collimating radiation in
a fan shaped beam 80 towards the detector housing. The detector
housing comprises an aluminium alloy cage and, in the embodiment
shown, contains two arcuate arrays of detector units 30, one at
each end, The detector housing includes rails along which the
detector units may be moved to different locations within the
housing. The housing shown could accommodate one or more additional
detector unit arrays if required.
[0079] FIGS. 12 and 13 show an apparatus for scanning steel pipe
82, having an internal diameter of about 234 mm and a wall
thickness of about 43 mm, to detect changes and flaws in the wall.
The pipe wall is surrounded by a layer of insulating material 84. A
support member 86 is clamped to the pipeline by means of clamps 88
hydraulically operated by arms 90. The support member also supports
rails 92 which support the support 70, detector housing 72 and
source unit 74. A motor 94 mounted on the detector housing is
operable to move the detector housing and source unit along the
rails and thereby to rotate the position of the source and
detectors around the pipeline. At each position, radiation emitted
by the source towards each detector in the two detector units forms
a number of radiation paths through the pipe wall and insulation
equal to the number of detectors, which in this case is
(19.times.2)=38 separate paths which can be scanned at the same
time. When the apparatus is rotated to a different position along
the rails, a further 38 paths can be scanned. Data in the form of
counts detected by the detectors is processed and stored by a data
processor housed in housing 96 located towards the top of the
support. Deployment of the detector units in the positions shown is
particularly suitable for scanning the pipeline walls and
insulation to detect flaws and changes between different locations
in the pipe.
[0080] In the detector housing shown, there is space for one or
more detector units to be placed in the central portion of the
housing. In that position, a detector unit would detect radiation
which has passed through the lumen of the pipe and its contents.
Use of a detector unit in such a position would therefore be
suitable for conducting tomography scans of the pipe and
contents.
[0081] FIGS. 14-15 show another embodiment of a scanning apparatus
according to the invention. The apparatus comprises two parts of a
hinged housing 102 which together form a clamp with jaws which can
be opened (FIG. 14) and closed (FIG. 15) around the pipe 82 by
operation of a hydraulic cylinder 98. When closed, the hinged
housings surround the pipe but are spaced apart from the surface of
the pipe. Rollers 100 contact the surface of the pipe and maintain
the spacing of the housing from the pipe. Housing 102 covers and
contains a detector housing for one or more arrays of detector
units and a source unit as described above. The source and detector
unit are mounted in a fixed relationship to one another and are
arranged to move along a track within the jaws so as to rotate
around the circumference of the pipeline. Adjustable grippers 104
are present either side of the pipe which are operable by means of
a hydraulic cylinder 108 to grip the pipe and centralise it within
the space between the jaws and the pipe. When the hinged housing is
closed around the pipe and centralised by grippers 104, the source
unit and detector housing are rotated around the pipe so that
density information can be acquired by the detectors at a plurality
of angular locations around the pipe. The data is then processed to
produce a tomography image or an indication of one or more
properties of the pipeline at different locations around the path
of the scanning operation. When sufficient data has been acquired,
the housing is opened and moved to a different location along the
pipeline for new scanning data to be acquired.
* * * * *